Fix sleep.go and make it coreutils ready

Following fixes are made:
* Better parsing (it's automatically now via time.ParseDuration)
* Better argument handling
* Current version was not building, fixed.
* Help and Version added (albeit version can be changed, it just a copy)
* Gnu Coreutil ready. That means the following works now:

./sleep 1s 3s 5s (will sleep for 8s)
./sleep 3s 1h (will sleep for 1 hour and 3s)
Merge pull request #13 from fatih/patch-1

Fix sleep.go and make it coreutils ready